1) Field of the Invention
The present invention relates to a process and an apparatus for continuously producing polyester type polymers such as polybutylene terephthalate and polyethylene terephthalate.
2) Related Art
Since polyethylene terephthalate (hereinafter referred to as PBT) resins are excellent in the crystallizing characteristic and also excellent in mechanical properties, electric characteristics and heat resistance, they have been used for applications such as electric machines, electronic parts, mechanical parts and automobiles and their demand has been increased steadily.
Heretofore, for the general PBT production process, a terephthalic acid alkyl ester comprising dimethyl terephthalate as a main ingredient and a glycol comprising 1,4-butanediol (hereinafter referred to as BD) as a main ingredient are placed at an appropriate ratio in a mixing vessel, a transesterification catalyst is added and conditioned and then they are sent to a transesterfication reaction vessel set to a predetermined reaction temperature by a pump. In the transesterification reaction, two or three stirring vessels with stirring blades are disposed in series and methanol formed as reaction by-products, and tetrahydrofuran (hereinafter referred to as THF) formed by decomposition of the methanol formed as reaction by-products and BD and water are separated in a distillation tower. Then, a polymerization catalyst is added and the process proceeds to the polymerizing reaction step. At first, vertical stirring vessels or horizontal stirring vessels are disposed in plurality for the prepolymerization step and, further, a horizontal stirring vessel is disposed as a final polymerization step.
For continuous polycondensation process for polyethylene terephthalate, etc. in a relatively low viscosity range, operated under subatmospheric pressure, a plurality multi-tray type, columnar reactors or a plurality of vertical complete mixing type stirring vessels are used in series as disclosed in JP-A-48-7090. Oligomers of low polymerization degrees formed by esterification reaction or transesterication reaction are continuously fed to one end of such reactors to successively proceed the polycondensation reaction down to the downstream tray or while transferring the oligomers from one stirring vessel to another.
In this connection, the present inventors proposed an apparatus for continuously producing polybutylene terephthalate, which comprises a first reactor for reacting an aromatic dicarboxylic acid comprising terephthalic acid as a main ingredient or a derivative thereof with a glycol comprising 1,4-butanediol as a main ingredient, thereby producing an oligomer with an average degree of polymerization of 2.2 to 5, a second reactor for polycondensating the oligomer from the first reactor, thereby preparing a low polymerization product with an average degree of polymerization of 25 to 40, and a third reactor for further polycondensating the low polymerization product from the second reactor, thereby producing a high molecular weight polyester with an average degree of polymerization of 70 to 130, or further a fourth reactor for further polycondensing the polyester from the third reactor to an average degree of polymerization of 150 to 200, thereby producing a high molecular weight polyester with good heat stability and excellent hydrolysis resistance, reactors without any stirrers by an external power source being used for the first and second reactors (U.S. patent application Ser. No. 09/642,587), parts of which are incorporated herein by reference.
In the apparatus, the second reactor is an approximately cylindrical vessel type, flow reactor in a double cylinder structure having an inner cylinder opening in the vessel and an inlet for the process solution at the lower part of the double cylinder structure, the process solution passing through tubes of a shell and tube type heat exchanger provided on the outside of the inner cylinder of the double cylinder structure and thereby heated to a predetermined temperature and passed upwardly to the level of the inner cylinder opening and then flowing down through the inner cylinder while the process solution is stirred with a plurality of doughnut-type trays provided on the inside wall of the outer cylinder, and the vessel is provided with an outlet for volatile matters and reaction by-products at the upper part thereof. The present inventors have found that short pass and thermal decomposition reaction of the process solution admit of improvement.